Conventionally, among optical components used in a DWDM (Dense Wavelength Division Multiplexing) system an optical component using a planar lightwave circuit (PLC) comes into practical use in a splitter or the like and is regarded as an up-and-coming device in view of high controllability and stability. With an eye toward development of the DWDM system in future, optical circuits using a PLC to serve as a dynamic gain equalizer, a tunable dispersion compensator and the like have been already developed.
Generally used to exhibit variable characteristics in these optical circuits is the thermo-optic effect. The thermo-optic effect is such a phenomenon that the refractive index of a silica glass or the like varies depending on the temperature. When the refractive index of an optical waveguide is changed with the thermo-optic effect, the effective refractive index of the temperature-changed portion of the optical waveguide is changed, which causes a change in phase of light propagating therethrough. With use of this, a variable waveguide interferometer is configured thereby to realize the variable characteristics.
Here, description is made about a configuration using a conventional dynamic gain equalizer with reference to FIGS. 14(a), 14(b) and 15. FIG. 14(a) shows gain profiles of an optical amplifier and a dynamic gain equalizer relative to wavelengths in the configuration of the conventional dynamic gain equalizer and FIG. 14(b) shows gain flattening profiles relative to the wavelengths.
In FIGS. 14(a) and 14(b), the dynamic gain equalizer manufactured with a PLC using the thermo-optic effect was used. The gain profile of the optical amplifier shown in FIG. 14(a) was subjected to flattening by the dynamic gain equalizer, which result is shown in FIG. 14(b). The insertion loss in this case was 7.0 dB.
FIG. 14(b) shows that the gain profile of the optical amplifier was flattened at a practical level for TE (Transverse Electro) polarization beam. However, for TM (Transverse Magnetic) polarization beam, the profile was not flattened and there occurred an extremely large PDL (Polarization Dependent Loss) of 4.5 dB or more. This PDL may be 10 dB or more depending on the wavelength characteristic of the optical amplifier, which sometimes becomes a significant obstacle to practical application. One of reasons for the large PDL is that heating by a heater in the dynamic gain equalizer causes anisotropic stress within the PLC and this stress produces a birefringence in the optical waveguide, which results in causing polarization dependence of the propagation characteristic.
In order to reduce the PDL, an optical circuit device 1000 shown in FIG. 15 is used (see e.g. non-patent document 1). FIG. 15 shows a configuration of the optical circuit device 1000. The optical circuit device 1000 includes an optical circuit 1010 of PLC, a polarization beam splitter/combiner 1020 and PMFs (polarization maintaining optical fibers) 1031 and 1032. The optical circuit 1010 and the polarization beam splitter/combiner 1020 are connected via the PMFs 1031 and 1032. The polarization beam splitter/combiner 1020 is connected to a circulator 200 via an SMF (single mode optical fiber) 300.
Light passing through the circulator 200 is first input to the polarization beam splitter/combiner 1020 via the SMF 300 and split into two polarization beams having polarization planes orthogonal to each other (for example, TE polarization beam and TM polarization beam in FIG. 15). One split beam (TM polarization beam in FIG. 15) has the polarization plane rotated 90 degree by rotation of the PMF 1032 and is input to the optical circuit 1010 (as TE polarization beam in FIG. 15). Then, the beam output from the optical circuit 1010 is combined at the polarization beam splitter/combiner 1020. The other split beam (TE polarization beam in FIG. 15) is input to the optical circuit 1010 while maintaining its polarization plane. Then, the beam output from the optical circuit 1010 has the polarization plane rotated by 90 degree and combined at the polarization beam splitter/combiner 1020. In other words, as there exists only one type of polarization beam in the optical circuit 1010 (only TE polarization beam in FIG. 15), PDL does not occur in principle.
Non-patent document 1: Doerr, “An Automatic 40-Wavelength Channelized Equalizer”, IEEE Photonics technology Letters, vol. 12, No. 9, September 2000, p. 1195